The present invention relates generally to sensors, including accelerometers, and finds particularly efficacious application in implantable cardiac pacemakers as a sensor for generating an electrical output signal indicative of a patient's activity.
The use of piezoceramic cantilevered beams is well known in the art of cardiac pacing, as are the equations which govern their response characteristics. Conventional electrical and mechanical connection of such a beam is typically accomplished by clamping onto the short edge of the beam to produce a cantilever configuration defining an overall beam length.
U.S. Pat. No. 4,140,132 to Dahl describes piezoceramic material in a physical activity sensor or accelerometer. An elongated piezoelectric cantilevered element is disclosed as having a weighted mass on one end of the element and being enclosed within an implanted cardiac pacemaker.
U.S. Pat. No. 4,896,096 to Nilsson describes an activity sensor of the flexural type, where a piezoelectric element is composed of two individual piezoceramic parts arranged side-by-side and that are oppositely polarized.
U.S. Pat. No. 5,235,237 to Leonhardt discloses a piezoceramic bending beam accelerometer enclosed within a housing where surface mount technology is employed. One end of the packaged accelerometer is clamped down within an enclosed package.
U.S. Pat. No. 4,653,326 to Danel et al. describes an accelerometer capable of measuring a component of acceleration by means of a variable capacitance capacitor.
U.S. Pat. No. 5,031,615 to Alt discloses a pacemaker employing an accelerometer comprising a miniaturized mechano-electrical converter or transducer formed in a semiconductor device.
U.S. Pat. No. 5,044,366 to Alt discloses a cardiac pacemaker for implantation in a patient that utilizes a pair of sensors.
U.S. Pat. No. 5,425,750 to Moberg discloses a multi-axis physical activity sensor for use with a rate-responsive pacemaker and a method for fabricating same.
The inventions disclosed in preceding references are all incorporated by reference herein in their respective entireties.
At least some of the inventions disclosed in the preceding references possess certain disadvantages. For example, the beam connection to the package or pacemaker shield may become a dominant factor in determining the sensitivity of the accelerometer when employing a bonding medium of either solder or conductive epoxy. When bonding, the medium may bleed onto the beam and result in a reduced effective net length of the beam as well as attenuation of piezoceramic sensitivity. Hence, the bonding step can adversely affect the overall beam performance and contribute to manufacturing yield loss. Additionally, many bonding methods require complex and expensive packing techniques to ensure a robust design.
Some prior art pacemaker accelerometers suffer from excessive mechanical fragility to the extent that the center cantilever beams thereof break, fracture or otherwise fail when the pacemakers are dropped onto hard surfaces from heights of only a few feet or inches. Finally, many prior art pacemaker accelerometers provide relatively low voltage output signals, and thus may provide output signals having unacceptably low or marginally acceptable signal-to-noise ratios.
At least some of the above-described devices described in the foregoing references may be modified advantageously by employing the teachings set forth below concerning the present invention.